Helium is the gas of choice for cold spray forming (CSF) process. However, the use of helium is economically prohibitive without sufficient helium recovery.
Generally, high velocities are necessary to accelerate the CSF powder towards the work piece. At 5 mole % nitrogen in helium, the sonic velocity will drop 8%. If the nitrogen concentration increases to 20 mole %, then the sonic velocity will reduce 33%. If a heavier impurity such as carbon dioxide reaches 20 mole % with the balance helium, then the sonic velocity will be reduced 43%. High gas velocities possible with pure helium are a desirable physical property regardless of the specific CSF application.
TABLE 1Sonic Velocity as a Function of Gas CompositionCarbonSonicHeliumNitrogenWaterDioxideVelocityMole %Mole %Mole %Mole %Temp. (F.)(Ft/sec)1000001003958955001003467901000100312085150010028598020001002653950501003645950051003225900010100278885001510024898000201002267
CSF is a newly developed technology that as of this writing has not been made commercial. CSF can be compared to thermal spraying (TS) with a primary difference being the nozzle gas temperature. TS uses particle velocity combined with thermal heat to form a coating on a work piece. A description of both processes will show a problem associated with TS that is solved with CSF and why helium was not used before and is the gas of choice for CSF.
FIG. 1 shows a schematic of the equipment enclosure for CSF and TS. One TS application is plasma spray. Passing gas through an electric arc inside nozzle 14 forms the plasma. Thus, for TS, nozzle 14, must be water cooled or contain refractory to permit high temperatures. The expected life for nozzle 14 is usually less than 100 hours. Gas and powder pass through nozzle 14 to form spray pattern 16. Typical nozzle gases could be a mixture of argon and hydrogen. In spray pattern 16 the hydrogen will combust to add additional heat to the powder. The powder will partially or completely melt in spray pattern 16 before hitting work piece 18 and forming a coating on work piece 18. Care must be taken that work piece 18 does not become too hot or the coating applied too thick. If the coating is too hot or applied too thick, then the coating will crack upon cooling. Care must also be taken in selecting the powder particle size. If the particle size is too small, then losses from vaporization will be economically prohibitive. Spray pattern 16 uses gas velocity and density to accelerate the particle at work piece 18. High temperatures present in spray pattern 16 decrease gas density which minimizes the impact of gas velocity on particle velocity. TS particle velocities of up to 200 m/s could be expected. Helium can provide higher gas velocities but the density would be substantially lower.
TS may require that a separate fluid be used to cool work piece 18. The separate fluid could be liquid carbon dioxide or water. Air is also passed through enclosure 12 through gas inlet 22. A high volume of air passes over work piece 18 and removes excess powder that did not adhere as the coating. The air and powder exhaust from enclosure 12 through gas discharge port 20. If helium were used in nozzle 14, then using air to sweep work piece 18 would make helium recovery and purification difficult and expensive.
CSF differs from TS in that at ambient temperatures the powder can be accelerated with helium to about 1000 to about 1200 m/s in nozzle 14 to work piece 18. CSF temperatures in nozzle 14, typically less than about 400° F., allows the use of particulate less than 20 micron in size and containing volatile alloying elements. The high velocities capable with helium give the particles sufficient energy to fuse into a coating when striking work piece 18. The resulting coating does not cause work piece 18 substrate to change as could happen if it was exposed to TS temperatures. Helium is also passed through inlet 22 to sweep over work piece 18 and remove excess powder. The helium and powder discharge from enclosure 12 through vent 20 to helium recovery and purification equipment. In CSF, helium ultimately serves two functions. One, it accelerates the coating powder, supplying kinetic energy. Two, it serves as a clean sweeping gas to clean the work piece of extraneous particles.
No known helium recovery system is believed to exist for CSF. The absence of helium recovery systems for CSF is not surprising because current CSF processes are lab scale and use small quantities of helium. However, other processes that use larger volumes of helium have helium recovery systems.
U.S. Pat. No. 5,377,491 discloses a coolant gas recovery process for a fiber optic cooling tube that uses a vacuum pump/compressor to remove cooling gas from the cooling tube, remove particulate and contaminants and then return the coolant gas to the fiber optic cooling tube. Purification equipment such as pressure swing adsorption, dryer and membrane are discussed with respect to removing water and oxygen, with the maximum quantity of oxygen in the range of 1 to 50 mole percent, and the cooling tube required to cool gas at 0 to 150 psig.
U.S. Pat. No. 4,845,334 discloses a plasma furnace gas recovery system where the gas exits the furnace at high temperature (˜700C.) and low pressure (<2 psig). The discharge gas is cooled and then followed by particulate removal equipment. The particulate free gas is then compressed, filtered again and then dried. The dry, compressed helium is then recycled back to the furnace at pressure using gas flows and pressures of 150 SCFM and 100 psig via an oil flooded screw machine.
U.S. Pat. No. 5,158,625 discloses a process for removing helium from a metal hardening (quenching) chamber, purifying the helium and compressing the helium. The quenching chamber was described as 10 M3 with helium at 2.5 bar absolute (875 SCF of helium). Helium and impurities may be recovered from the hardening furnace through a vacuum pump. Down stream of the vacuum pump the helium plus impurities would be compressed and stored in one receiver. Once all of the desired helium from the hardening furnace was removed, then helium with impurities was passed through a membrane, dryer, PSA or catalytic oxidation of hydrogen to remove oxygen and water from the process. The purified helium is then compressed again and stored at pressure in another receiver until the next hardening cycle starts. The above process uses higher than atmospheric pressures in the quenching chamber to increase the helium density and thus improve the heat transfer capability.
The prior art does not teach or suggest the recovery and purification system comprised of three continuous loops involving the strategic placement of the purification equipment. Further, each loop has its own separate function. In addition to purification and recovery, the current invention is capable of pressurizing the helium to achieve the requisite sonic velocity.